1. Field of the Invention
The present invention is directed to inhibiting or preventing popcorn polymer growth or formation, particularly such growth or formation in organic material. The desired result is effected by treatment with one or more esters of inorganic acids. This treatment is conducted with an amount of such inhibitor sufficient to prevent, inhibit, retard, or stop popcorn polymer growth.
2. Description of Background and Other Information
Popcorn polymers are known to form from all manner of organic material, particularly olefinically unsaturated monomers, including olefins and diolefins; especially susceptible are the conjugated diolefins, e.g., butadiene and isoprene, and vinyl compounds, e.g., styrenes and acrylates. Known as popcorn polymers because they resemble popped corn, these polymers are also referred to in the art as sponge polymers, granular polymers, cauliflower-like polymers, nodular polymers, fluffy polymers, proliferous polymers, and crusty polymers.
Popcorn polymer has been considered to occur from spontaneous monomer polymerization. It can occur in both liquid phase and vapor phase, and at any stage of use or handling of the monomer, e.g., recovery, separation, manufacturing, purification, storage, etc. High concentrations of monomer are particularly advantageous for its formation.
Specifically, it appears that the presence of one or more initiators--e.g., water, oxygen, hydrogen peroxide--results in the formation of popcorn polymer "seeds" in the organic material. The seeds themselves then perpetuate polymerization, without further requiring an initiator and/or a crosslinking agent; they serve as sites for further polymerization.
As the particular mechanism, it is believed that monomer diffuses through the surface of the growing polymer mass, and is added to the polymer at the center thereof. For this reason, such polymerization is referred to as occurring "from the inside out."
Consequently, there is continued incorporation of monomer into the polymer phase, leading to buildup of the popcorn polymer. The result is a hard polymeric foulant, which can cause serious equipment and safety concerns if left unchecked.
A particular problem attendant upon popcorn polymer formation is its extreme resistance to deactivation, once present in a system. Some of the seeds become attached to the processing and handling equipment, and cannot be readily removed by mechanical means; moreover, being insoluble in most common solvents, they are virtually impossible to wash out by use of such solvents.
Even after equipment and storage facilities have been cleaned thoroughly, residual particles of popcorn polymer remain, and promote unwanted polymer growth. Trace particles remaining in the equipment will stay active for long periods without the presence of monomer, and serve to initiate polymerization when once again contacted therewith.
Different inhibitors are known for use against popcorn polymer formation. Examples of these are the following: t-butylcatechol; sodium nitrite, as disclosed in LIU, "Plugging-Up of Equipment by Self-Polymerization Butadiene Production and Its Prevention," China Synthetic Rubber Industry, 11(5) 357-360 (1988); carbon disulfide and elemental phosphorous, as disclosed in HASKELL, U.S. Pat. No. 4,404,413, which also refers to hydrogen sulfide, to ethane-, propane-, and hexane-thiol, and to ethyl disulfide as being known in the prior art; N,N-dialkylhydroxylamines, as disclosed in TOKAI ELECTRO-CHEMICAL CO., Japanese Kokai No. 66,223,003, as well as in LIU et al., "Determination of Traces of Diethylhydroxylamine Inhibitor in C.sub.5 Fraction by Gas Chromatography," China Synthetic Rubber Industry, 12(6), 408-410 (1989), and in ALBERT, U.S. Pat. No. 3,148,225, the latter of these also referring to nitrites, nitroso compounds, NO.sub.2, N.sub.2 O.sub.3, phenolic compounds, sulfur, aromatic amines, and hydroxylamine as being known in the prior art; trialkylamine oxides, as also disclosed in TOKAI ELECTRO-CHEMICAL CO.; N-hydroxymorpholine, used in conjunction with N,N-dialkylhydroxylamines, as disclosed in WHITON et al., U.S. Pat. No. 3,265,752, or in conjunction with N-hydroxypiperidine, as disclosed in McCOY et al., U.S. Pat. No. 3,265,751; adducts of phenols and hydroxylamines, as disclosed in ALBERT et al., U.S. Pat. No. 3,493,063; reaction products of nitrous acid and 1,3-dichlorobutene-2 or diisobutylene, as disclosed in BENJAMINS, U.S. Pat. No. 3,560,577, which also refers to nitrogen dioxide, the addition product of 1,3-dichloro-2-butene and nitrogen dioxide, and ion-exchange resin containing nitrite ions, as being known in the prior art; butyraldoxime, as disclosed in KEOWN, U.S. Pat. No 2,947,795; and nitrogen tetroxide-diisobutylene addition products, as disclosed in COLBERT, U.S. Pat. No. 3,175,012.
Those inhibitors known in the prior art are generally effective in stopping the formation of popcorn polymer seeds. However, they are only minimally effective in stopping the growth of seeds already in existence. Further, such inhibitors which are relatively heavy will work in liquid phase, but are of little or no use in vapor phase, because in this state their weight hinders their distribution.
It has been discovered that esters of inorganic acids will inhibit popcorn polymer formation. Specifically, such compounds are characterized by at least one substituted or nonsubstituted hydrocarbyl group and the anionic moiety of an inorganic acid.
For instance, inhibitors of the invention can be used to treat organic material wherein such popcorn polymer formation occurs. Specifically, these inorganic acid esters may be admixed with, or added to, such organic material during the use, handling, or storage thereof.